The invention relates to a color display tube comprising, in an evacuated envelope, means for generating a number of electron beams, a display screen having regions luminescing in different colors when struck by the beams, and a color selection electrode, disposed between the beam generating means and the display screen, which has apertures for passing the electron beams and associating each electron beam with luminescent regions of one color, said color selection electrode being coated at least on the side remote from the display screen with a layer which comprises a heavy metal. The invention further relates to a method of manufacturing such a color display tube.
Heavy metal is understood to mean hereinafter a metal or an alloy of a metal having an atomic number higher than 70.
A color display tube of the type mentioned in the opening paragraph is known, for example, from GB-A No. 2 080 612.
This Great Britain Patent Application describes that a color display tube having a color selection electrode which on the side remote from the display screen is coated with a layer of a material comprising a heavy metal, during operation shows considerably less local or overall doming. Such doming would deteriorate the color purity of the color display tube.
In fact, a large number of the electrons, on their way to the display screen, is intercepted by the color selection electrode, sometimes termed shadow mask, and causes local or entire heating of the shadow mask and, hence, doming. A layer with heavy metal on the side of the color selection electrode where the electron beams are incident has for its effect that the electron beams are refracted more strongly beyond the apertures and give less rise to heating and doming.
It is also stated in the above-mentioned literature reference that, depending on the increase of the thickness of the heavy metal-containing layer, the possibility of the occurrence of loose particles in the tube increases. These loose particles may give rise inter alia in the electron gun to high voltage flashovers and on the display screen to black spots in the displayed picture.
One of the objects of the present invention is to prevent the occurrence of loose particles in the tube by the action of the electron beams on the heavy metal-containing layer.
Another problem occurring in the color display tube is the following. The color selection electrode owes its rigidity for a considerable part to its curvature. Increasing the radius of curvature causes a weakening of the mask. In the present-day developments of the color display tube, there is a tendency to make the screen less convex or even flat. It is then desired to make the color selection electrode more rigid.
Another object of the present invention is to provide a more rigid color selection electrode.